This invention relates to solid state photodetector devices.
Photoconductivity is the name given to the increase in electrical conductivity which occurs in a nonmetallic solid when it is exposed to electromagnetic radiation. This increase in conductivity occurs as a result of the additional free charge carriers (electrons or holes) which are generated when photons are absorbed in electronic transitions. The rate at which the free carriers are generated and the length of time they persist in a conducting state determines the amount of the change in conductivity.
In order to be absorbed, a photon must have a quantized energy which is equivalent to or greater than the energy bandgap which is characteristic of the absorbing material. This effect has been used to advantage to construct photodetector which are sensitive to particular frequencies of light. In a semiconductor, for example, a photon may be absorbed if it has an energy corresponding to the bandgap between the valence band and the conduction band of the semiconductor. Thus a photodetector sensitive to light of a particular frequency may be achieved by applying an electric field across a semiconductor material having the appropriate bandgap. When light of the predetermined frequency is directed onto the semiconductor, photons are absorbed, thereby creating electron-hole pairs which contribute to the conduction process. The extent of the resulting change in the conductivity of the material will indicate the amount of light which is present.
Unfortunately, however, the spectral range of light which can be detected by semiconductor detectors has heretofore been limited because of the quantized values of the energy bandgaps between the valence and the conduction bands is such materials. Semiconductor compounds formed by combinations of elements from columns III and V of the periodic table, for example, have been utilized in such detector configurations, but such III-V detectors with E.sub.g .gtoreq.0.62 eV are not responsive to light with wavelengths greater than 2 .mu.m. It would be very advantageous to provide a detector structure utilizing semiconductor materials which can be adjusted to detect light in a particular wavelength region, rather than having to select materials with an appropriate bandgap but with undeveloped or uncertain processing technology.